1. Field of the Invention
The present invention relates to a pressing mold which can be advantageously used for pressing a steel sheet having a high strength, such as a high tensile steel sheet or the like.
2. Description of the Related Art
In recent years, high tensile steel sheets are used for the purpose of improvement of safety and lightweight of a car body in the field of vehicle production.
A high tensile steel sheet, which has a high tensile strength and other mechanical strengths, requires higher pressing force in press working than conventionally used general press steel sheets. Therefore, in the press working of the high tensile steel sheet, the abrasion of the pressing mold is likely to rise due to the higher pressing force, which results in reduction of the lifetime of the mold.
To suppress the mold from being worn, there has been proposed a method of increasing the surface hardness of the mold by forming a hard coating film, such as coating film made of TiC (titanium carbide), TiCN (titanium carbonitride), on a surface of a mold body (see, for example, Japanese Patent Unexamined Publication No. HEI 6-145960).
As a method of forming a hard coating film, such as TiC coating film, there is known CVD (Chemical Vapor Deposition) method or PVD (Physical Vapor Deposition) method.
A coating film formed by CVD method is known to have a higher adherence than that formed by PVD method. Accordingly, CVD method is conventionally used to form a TiC coating film which has relatively low adherence with a mold body.
However, the coating film formation by CVD method requires a processing temperature of 1000° C. or greater, which causes a problem that the mold body is subjected to distortion or thermal deformation in the coating treatment. Consequently, a mold formed with a coating film by CVD method requires another operation of correcting the dimension of the mold after being formed with the coating film.
On the other hand, in the case of forming a coating film by PVD method, the temperature of the coating treatment does not need more than 500° C., which is lower than tempering of high speed steels or high tempering of dies steel. Consequently, the mold body is subjected to a relatively small distortion and thermal deformation. Thus, a mold formed with a coating film by PVD method has a favorable merit that no dimension correction is required after the coating treatment.
However, the TiC coating film formed by PVD method is liable to separate from the surface of the mold body because the adherence of the TiC coating layer is insufficient. Consequently, the durability of the mold is short. Among hard coating films formed by PVD method, further, a TiCN coating film is known to have a better adherence than the TiC coating film. However, the TiCN coating film is less resistant against abrasion due to a low slidability.
As a method of totally evaluating the quality of a coating film formed on a surface of a mold body, such as durability and slidability, an evaluation method using a bead drawing characteristic has been known, which will be described in detail in examples later.
A bead drawing characteristic is, as shown in FIG. 4, determined by placing a steel sheet between a male mold 30 and a female mold 31, moving the both molds 30 and 31 to each other to thereby press the steel sheet, drawing the steel sheet at a constant speed, measuring a pressing load P and a drawing load F when the steel sheet is broken off. The durability and slidability of a coating film formed on the mold surface can be calculated based on thus measured bead drawing characteristic. Specifically, a mold which provided a higher pressing load P and a higher drawing load F may be judged to be provided with a coating film having higher slidability and durability.
The conventional pressing molds provided with a hard coating film formed by PVD method have poor bead drawing characteristic.